Some 200 protesters gathered near the entrance of the convention centre near Tokyo, calling for the government-backed DSEI Japan exhibition to be shut down as they regarded it as an affront to the nation's pacifist constitution.
Worried by increased Chinese military activity in the East China Sea and North Korea's ballistic missile advances, Japan has increased defence spending over the past seven years to around $50 billion annually, purchasing advanced US stealth fighters, missile defence interceptors and radar systems.
"Technology is advancing quickly and our equipment can't cope against things such as hypersonic warheads and drones," Gen Nakatani, a former defense minister and senior ruling Liberal Democratic Party lawmaker told Reuters at the arms show.
"Innovation is happening around the world and through an exchange of that Japan will be able to keep up," he added.
China spends more than three times as much as Japan on defence, while recent North Korean advances threaten to make Japan's new missile defences obsolete before they are deployed.
Prime Minister Shinzo Abe's government in 2014 abolished a decades-long ban on foreign military exports in a bid to cut procurement costs by allowing Japanese companies such as Mitsubishi Heavy Industries to widen their production base.
Yet in the more than five years since that ban ended Japan has largely failed to make inroads overseas, hobbled both by a lack of experience and concern that the reputational risk of selling arms could hurt other more profitable businesses.
There is still abiding foreign interest in tapping Japanese technology for military use, with companies such as Lockheed Martin Corp , Raytheon Co and BAE Systems PLC all looking for new partnerships in Japan.
"There is a great deal of interest internationally in seeing what Japan has to offer the world," said Alex Soar, International Development Director at Clarion Events which organized the show covering land, air and naval equipment.
Abe's government faces opposition at home to policies that some Japanese people fear could erode the pacifist constitution and herald a return to the militarism that devastated the country in World War Two. "Producing more weapons is not going to make us safer. Japan has to rely on diplomacy," said one of the protesters, who only gave her first name Takako.